Ultra short optical pulses can be used in a number of applications including optical information processing and data communication, optical probing with high temporal resolution, laser surgery, and material processing. In particular, recent advances in optical data communication with data rates up to 2.5 Gbits/s or higher demand compact ultra fast light sources with low maintenance, high reliability, and low cost.
Fiber lasers have been developed as a new generation of compact, inexpensive and robust light sources. In essence, a fiber laser is an optically-pumped resonator with a doped-fiber as the gain medium. As the gain exceeds the total optical loss in the resonator, a laser oscillation can be generated. Many different dopants can be used to achieve laser oscillations at different wavelengths. Atomic transitions in rare-earth ions can be used to produce lasers from visible wavelengths to far infrared wavelengths (e.g., 0.45 .mu.m.about.3.5 .mu.m). Er-doped fiber lasers for producing optical pulses at 1.55 .mu.m are particularly useful for optical fiber communication since the optical loss in the commonly used silica fibers is minimum at about 1.55 .mu.m.
Mode-locked fiber lasers can use various cavity is configurations such as linear, ring, and figure-eight geometries. See, for example, U.S. Pat. Nos. 5,008,887 to Kafka et al., 5,513,194 to Tamura et al. However constructed, a mode-locked fiber laser is configured to have multiple longitudinal modes that oscillate simultaneously. A mode-locking mechanism is implemented in the resonator to synchronize the phases of different modes in such a way that the phase difference between any two adjacent modes is a constant. These phase-locked modes constructively add to one another to produce a short pulse.
Two common mode-locking schemes are active mode locking and passive mode locking. Active mode locking modulates either the amplitude or the phase of the intracavity optical field at a frequency equal to one or a multiplicity of the mode spacing. Active mode locking can be implemented by using intracavity electrooptic and acoustooptic modulators.
Alternatively, passive mode locking uses at least one nonlinear optical element inside the resonator to produce an intensity-dependent response to an optical pulse so that the pulse width of the optical pulse exiting the nonlinear element is reduced. Compared to active mode locking, passive mode locking can be used to achieve shorter pulses and therefore can be used advantageously to produce ultra short light sources. Commonly used passive mode locking techniques include saturable absorbers, nonlinear fiber-loop mirrors (e.g., figure eight fiber lasers), and intensity-dependent nonlinear polarization rotation. See, Richardson et al., Electronic Letters, Vol. 1, pp. 542, 1991 and Tamura et al., Electronic Letters, Vol. 28, 2226, 1992.
Mode-locked fiber lasers are much more compact and reliable than solid-state mode-locked lasers such as color-center lasers and Ti-Sapphire lasers. Compared to modelocked semiconductor lasers with typical pulse widths of 10-20 ps and peak power of milliwatts, mode-locked fiber lasers can generate shorter pulses with higher output peak power.